Method and apparatus for electron beam control

ABSTRACT

The beam of charged particles of a luminescent screen tube is displaced under the combined action of two orthogonal scans, the spot being constrained to follow guiding strips formed on the luminescent screen surface and responsive to the position of said spot by modulating the control of one of the two scans by means of an error signal which is deduced from the indications supplied by the guiding strips. By means of continuous correspondence control, the spot is initially guided over an entire line entrance surface towards the beginning of the path to be followed by said spot before it passes onto the screen proper.

Lejon States Patent [191 Mar. 26, 1974 METHOD AND APPARATUS FOR ELECTRON BEAM CONTROL [22} Filed: Oct. 16, 11972 [21] Appl. No.: 298,083

OTHER PUBLICATIONS RCA Technical Note No. 229 Received Jan 5, 1959.

Primary Examiner-Robert L. Richardson Attorney. Agent, or Firm-William R. Woodward: Flynn & Frishauf [57] ABSTRACT The beam of charged particles of a luminescent screen tube is displaced under the combined action of two orthogonal scans, the spot being constrained to follow guiding strips formed on the luminescent screen surface and responsive to the position of said spot by modulating the control of one of the two scans by means of an error signal which is deduced from the indications supplied by the guiding strips. By means of continuous correspondence control, the spot is initially guided over an entire line entrance surface towards the beginning of the path to be followed by said spot before it passes onto the screen proper.

27 Claims, 17 Drawing Figures PATEiUEU 1974 SHEET 05 OF 15 PATENTEUMARZS 1974 3.800.072

SHEET '07UF15 SHEET :09 or 15 HG. H

SHEET 10 0F 15 PMENTEBI-EARZB I9 3800072 snw an m 15 4 SHEEY 1SOF15 METHOD AND APPARATUS FOR ELECTRON BEAM CONTROL This invention applies to luminescent screen tubes in which light signals are generated by impact of a beam of charged particles, the displacement of which is carried out by a combination of two orthogonal scans obtained respectively by means of two electromagnetic or electrostatic fields which are also orthogonal.

Among the tubes of this type, mention can be made of the ion tubes or the cathode-ray tubes such as, for example, those employed in oscilloscope screens or more generally for the reproduction of images in television.

' In tubes of this type, the exact position of the spot at a given moment is defined only by the two horizontal and vertical deflection control systems, which entails the need to ensure that the trace of the spot on the screen, or television image, is subjected to a certain number of inevitable distortions under the action on the one hand of interference and on the other hand of inherent defects in the electronic system for controlling the two deflections.

If the particular case of color television is now examined, it is observed that the above-mentioned defects have even more serious consequences in regard to the quality of the image than is the case in monochrome television.

In fact, it is known that the tubes in widest use in color television are of the so-called shadow-mask type. In tubes of this type, the luminescent coating which lines the internal wall of the screen is made up of over a million luminescent dots, or luminophors, which are grouped together in triads each comprising one red luminophor, one blue luminophor and one green luminophor. At a distance of approximately mm behind the screen, there is placed a mask pierced with a number of holes corresponding to the number of triads of luminophors, namely approximately 400,000. The mask is placed in such a manner as to ensure that its holes are in register with the centers of each triad. Moreover, the neck of the picture tube contains three electron guns each corresponding to the transmission of information relating to one color and positioned within the tube in such a manner as to ensure that each electron gun is in direct line of access, through the holes of the mask, only to those luminophors which correspond to its color.

It is consequently apparent that any distortions which are inherent in monochrome systems have repercussions which in this case are increased by the existence of three beams relatively displaced in space and that these are attended by a number of further consequences which arise from the fact that any deviation from the theoretical operation of the mask produces an error in the restoration of the color sensation.

To these numerous defects can finally be added the fact that there is a loss in the shadow mask of 80 percent of the beam energy which is stopped and converted to undesirable heat at the time of passage of the beam through said mask as well as the considerable difiiculties involved in the technological development of a tube of this type.

It has already been proposed (as disclosed in particular in French Pat. No. 948,653) to control automatically the position of the spot on the screen of a television tube by comparing at each instant the real state of the scanning field with a standard reference field and by correcting the instantaneous position of the spot by means of a control loop driven by a signal obtained by comparison with an arbitrarily fixed reference value which must correspond to a predetermined position of the spot with respect to the fixed field.

Since the intensity of the spot is modulated as a function of time for the reproduction of an image, the error signal obtained is merged into the spectrum of the image modulation signal and cannot in practice be separated from the signal just mentioned, thereby making any correction illusory even when making use of the device for varying the gain of the correction amplifier as described on page 3, lines 21 to 32 of French Pat. No. 948,653.

Moreover, the system as described is also inoperative by reason of the fact that it presupposes perfect coincidence between the theoretical field represented by the reference conductive strips and the real field of the beam. In point of fact, even if a relative displacement of very small value occurs at the start of each line, the error which is accumulated after a number of lines results in displacement of the spot with respect to its reference field and the method becomes inoperative. In an alternative mode of execution (page 3 lines 33 to 49), it has certainly been proposed to accumulate the error signals read at the end of each line by making use of these signals themselves as vertical deflection signals. Unfortunately, this improvement is inapplicable in practice by reason of the dissymmetry which is present in all systems of optoelectronic deflection systems.

This invention is directed to a method for guiding the beam of a luminescent screen tube which makes it possible to overcome most of the defects mentioned in the foregoing by ensuring strictly accurate displacement of the spot along the lines of one of the two scanning fields, and which permits in particular excellent reproduction of the color images while being much more simple to manufacture than the shadow-mask tubes.

This method of guiding the beam of charged particles of a luminescent screen tube consists in displacing said beam under the combined action of two orthogonal scans, in controlling the spot to be followed in dependence on guiding means carried by the screen and responsive to the position of said spot by modulating the control of one of the two scans aforementioned by means of an error signal deduced from the indications supplied by said guiding means, and in first guiding said spot by continuous control over the entire surface of a line entrance zone before said spot passes onto the screen proper, towards the beginning of the path which it is intended to follow.

The lines of the horizontal scan are defined on the screen by guiding means which the spot is constrained to follow as virtual rails"; distortion is therefore no longer possible in the vertical scan, irrespective of any interference which may also be present or of defects which may be encountered either in the electronic circuits or the optical system of the tube. This result is obtained by virtue of the fact that the guiding means located on the screen are responsive to the passage of the spot, to which they react by emitting a continuous or periodic current (which can in any case be only a fraction of the beam itself) or an electromagnetic radiation. Inasmuch as the intensity or amplitude of the frequency of the signal thus collected is characteristic of the difference between the real position and the theoretical position (defined by the guiding means) of the spot at each instant, there is thus formed an error signal which serves to modulate the vertical scanning integrator control. The controlling action thus obtained is very fast by reason of the low L/R time constant which is chosen for the vertical scanning control.

In accordance with one essential property of the method which forms the subject of this invention, the spot is previously guided before each line entrance on the lumimescent screen proper towards the beginning of the rectilineal path which it is intended to follow. To this end, a line entrance zone extending downwards over the entire left-hand portion of the screen (which cannot be seen by the viewer) is lined on the inside wall of the picture tube with beam-guiding means, the signals of which are emitted under the action of the spot and applied to the vertical deflection control so as to constrain the spot to move towards the line entrance. In accordance with an interesting feature ofthis arrangement, the gain of said position control is increased by increasing the intensity of the beam during the last part of the line blanking signal. The presence in accordance with the invention of a line entrance zone over which the spot is guided by continuous control each time the beam returns to the beginning of its intended path makes it possible to overcome by simple means the major difficulties which were explained earlier and which arose in the cited prior art from the inevitable dissymmetry of the optoelectronic deflection system.

The method according to the invention applies generally to all types of screen scanning which are employed in practice, However, for reasons of simplicity, the following description will be limited to the method of scanning which is the most widely adopted in television, namely to two deflections respectively in the vertical and horizontal directions, in which the spot scans successively from left to right (as seen by the viewer) and from top to bottom a series of straight or so-called horizontal lines although these latter are in fact very slightly inclined downwards from left to right. However, it must remain clearly understood that the ensuing description is not intended to imply any limitation and the transpositions which are necessary in order to apply the invention can be made without any difficulty by those who are versed in the art.

In the particular and very important case in which, as a result of constant-pitch spatial distribution of the beam-guiding means, the signal collected is periodic and has a theoretical frequencyfin respect of a theoretical horizontal linear scanning velocity v, the difference between said valuefand the measured real instantaneous value f is utilized in accordance with the invention for the purpose of forming an error signal which is applied in this instance to the horizontal scanning control, with the result that the spot is caused to move along each line at the constant theoretical velocity v. In accordance with this very important characteristic feature of the method according to the invention, the distortions of the horizontal scan are therefore-also corrected irrespective of the causes of such distortions. This possibility which is offered by the invention is of considerable significance since it permits the construction of tubes having very wide angles of deflection and therefore of much smaller overall length, this being achieved by means of very simple correction circuits.

Finally, there are at the present time a certain number of cases, especially for the reproduction of color images, in which the luminescent screen tube employed contains a number of electron guns in which the beams are intended to travel along parallel traces on the screen. In this case, the method according to the invention is perfectly applicable in the following manner one of the beams, or so-called master beam, is caused to follow the guiding means carried by the screen and the other beams, or so-called slave beams, are constrained to follow the displacements of the master beam; to this end, the centering permanent magnets are simply set once and for all before the tube is put into service and in such a manner as to ensure that all the beams emitted by the different electron guns penetrate into the zone in which the magnetic scanning fields are set up as parallel as possible to the axis of the deflecting system and as close as possible to said axis.

This invention is also directed to a luminescent screen which is essentially provided over the entire luminescent surface proper of said screen with a series of guiding strips having a width substantially equal to the diameter of the spot and each corresponding to one of the lines of one of the scanning fields, said strips being formed by a succession of zones modulated in width with a pitch p1, any positional deviation of the spot with respect to one of said strips resulting in an error signal which produces by integration the signal which controls the other scan in order to cause displacement of said spot at each instant while strictly centered on the edge of said strip.

The use of guiding means distributed with a constant pitch gives rise to an error signal constituted by at least one amplitude-modulated carrier frequency. By virtue of this essential property, said error signal can be very readily distinguished from the video modulation, thereby providing a simple and elegant solution to the difficulties which, in the devices of the prior art hereinbefore described, resulted in confusions between the error signal and the video signal.

The rectilineal horizontal guiding strips are formed by a succession of zones modulated in width with a pitch pl and the scansion of the spot at the horizontal velocity v along each strip consequently produces a signal having a frequency fl v/pl and whose amplitude of greater or lesser value is characteristic of the position of the spot with respect to said strips; it is accordingly necessary to use only the amplitude variations of said signal as an error signal to control the vertical scan. In one particular case of appreciable significance,

I said modulated strips are constituted by a series of broken lines having a pitch pl. In all cases, the modulation of the collected signal at the frequency fl very considerably facilitates its identification.

In the most common designs, the scanning system is the type employed in ordinary television receivers, namely a combination of a horizontal deflection and a vertical deflection. In this particular case, the guiding strips are rectilineal and horizontal on the screen.

When the guiding strips are simple, the control is of the unilateral type and the error signal which results from the instantaneous deviation between the theoretical position of the spot defined by the guiding strip and its real position accordingly produces the vertical scanning control signal directly by integration so as to cause the spot to move horizontally at each instant while remaining strictly centered on the edge of the guiding strip.

When the guiding strips are made up of a conductive metal, said strips themselves serve as electrodes for the partial return of the master beam. In other Words, it is the more or less large fraction of the beam current intensity which traverses said strips and which is therefore characteristic of the vertical position of the spot with respect to said strips which is employed as an error signal. The second fraction of the master beam current is closed on the return electrode which is common to all beams which may be employed.

Broadly speaking, the collected signals derived from the reactions of the guiding strips to the scanning travel of the spot can be of different types: in particular, they may consist of a fraction of the beam current which is closed on said conductive strips, or an electromagnetic radiation such as ultraviolet radiation, for example, which is detected by a photoelectric detector and converted into a current having a frequency which is characteristic of the identity of the signal; in some cases also, the guiding strips are formed of a material which gives rise to a strong emission of secondary electrons which can be collected by means of a special electrode located within the tube in the vicinity of the luminescent screen, the current constituted by said secondary electrons being then employed as an error signal.

In all the examples hereinbefore described, the guiding means are constituted by a single modulated strip which is marked on the screen at the location of each horizontal scanning line. In other words, in these examples, control of the spot of the master beam which is made to follow these strips is carried out in a unilateral manner and it is well known in the technique of correspondence control systems in general that a method of this type gives rise to practical difficulties, especially when the intensity of the beam is modulated.

It is for this reason that, in other embodiments of luminescent screen tubes according to the present invention, the guiding means are constituted by a series of double strips, that is to say in which each strip has an upper guide and a lower guide spaced at a distance from each other which is smaller than the diameter of the spot in order that this latter may always be in contact with at least one of the two guides at the time of scanning. In these embodiments, the error signal employed which results from the positional deviation of the spot of the master beam with respect to said double strips is constituted:

either by the difference in the amplitudes of the signals corresponding respectively to the upper guide and to the lower guide of each strip, in which case said error signal is employed for modulating the vertical scanning control signal at a higher or lower level depending on whether it is either positive or negative, thus resulting in a much easier and much more accurate control since the error signal is alternately positive and negative;

or by the ratio of said amplitudes which must be maintained equal to unity, which offers the additional advantage of eliminating any component arising from the modulation of the beam intensity.

In accordance with a first alternative embodiment, the two guides of each double strip are constituted by continuous electrically conductive metallic strips; each of these two guides receives a variable fraction of the beam current as a function of the vertical position of the spot and the difference between these two current fractions collected in two separate return circuits is used to constitute the error signal which serves to modulate the input signal of the vertical scanning control integrator.

In another alternative embodiment, the two guides of each double strip are made up of width-modulated strips, the width of the upper guide of each strip being modulated with the pitch p1 and the width of the lower guide of each strip being modulated with the pitch p2. Under these conditions, when the spot of the master beam scans the double strip at the velocity v of horizontal linear displacement, said spot produces two families of signals having the respective frequenciesfl v/p1,/2 v/p2, the respective amplitudes of said signals being representative of the exact vertical position of the spot at each instant. It is therefore only necessary in accordance with the invention to form in accordance with known means the difference or the ratio of said amplitudes in order to constitute the error signal which serves to modulate at a higher or lower level the input signal of the vertical scanning control integrator. In this alternative embodiment, the collected signal which has two frequency components f1 and f2 is collected in the form ofa current only and the two guides of each strip when they are conductive are connected electrically to a single and common electrode for collecting the signal. In the most frequent case, the signal having the frequency components fl and fl is constituted simply by that fraction of the master beam current which is closed on the upper and lower portion of the guiding strips within the electrode which is connected to said strips, the other current fraction being closed on another electrode and this latter can advantageously be the return electrode which is common to the luminescent screen proper.

In further embodiments, it is an advantage to form said continuous strips of a substance which gives rise either to the emission of an electromagnetic radiation which is detected by a photoelectric cell located in the vicinity of the picture tube neck or of a substance which gives rise to the emission of two secondary electron currents having frequencies fl and f2 which are accordingly collected on an electrode placed in the vicinity of the screen. In either case, the current obtained in the single circuit contains frequency components f1 and f2 which can readily be identified and are employed for constituting the error signal as hereinbefore described.

In accordance with a further important feature of the present invention, the luminescent screen tube is provided within the entire line entrance zone on which the spot impinges each time the beam returns with means for guiding said spot towards the start or entrance of the line which is to be precisely described by said spot. In the most common case of a rectangular television screen with horizontal and vertical scanning, said line entrance zone extends over the entire height of the screen on the left portion of this latter as considered from the viewing side but outside the luminescent surface proper and concealed from the viewer by the casing of the television set.

In a first embodiment, said guiding means of the line entrance zone consist of continuous surfaces which are responsive to the presence of the spot and located on each side of the extensions of each horizontal and rectilineal guiding strips which constitute two sets of electrodes. Under the action of the spot, the surfaces aforesaid generate currents il in the case of that portion which is located above the guiding strip and i2 in that portion which is located below said guiding strip. It is therefore readily understood that, by making use of the difference between these two currents i1 and i2 which are collected at the two sets of electrodes as an error signal for modulating the vertical beam sweep, it is possible to obtain a fast vertical deviation of the beam in order that the spot should be very rapidly brought into the vicinity of the entrance of the line which it is intended to describe.

In another embodiment of said line entrance zone, the guiding means consist of non-continuous surfaces. The zone which is located on each side of the extensions of each horizontal rectilineal strip comprises blocks of surfaces arranged in alternate sequence and having a pitch p1 above said strips and a pitch p2 below these latter. Thus, as the spot of the master beam travels at the horizontal velocity v, so said spot produces signals having a frequency fl v/pl and f2 v/p2 such that the difference or ratio of amplitudes can be utilized as an error signal for modulating the vertical sweep or scan of the beam and the other fraction of the master beam is closed in a loop, for example by the common electrode of the luminescent screen.

For the reproduction of monochrome images, it is certainly possible to contemplate modes of application of the invention which consists ofa single electron gun, and in which the guiding spot is used at the same time for reproduction of the image; however, such a constructional design is not without difficulty since it calls for the presence ofa minimum signal which impairs the black. In the majority of cases, the invention is carried out by means of two electron guns, the beams of which are constrained to travel in parallel relation, the spot of the first gun or master beam being caused to travel on the non-luminescent guiding strip and the spot of the second gun or slave beam being caused to travel on a luminescent strip of the screen.

However, it is in its applications to the reproduction of color television images that the present invention finds one of its most interesting and advantageous applications. The invention is accordingly directed to a luminescent screen tube which is distinguished by the fact that the horizontal guiding strips are each associated with a triplet of three continuous horizontal and rectilineal strips of luminescent material each corresponding to one of the primary colors, the tube being provided with four electron guns in which the first or so-called master gun emits the guiding beam which is caused to follow the horizontal and rectilineal guiding strips and in which the three other electron guns or socalled slave guns each correspond to one of the three primary colors and emit beams which are caused to follow the displacements of the master beam so that the corresponding spots should travel in a direction parallel to the spot of the guiding beam while strictly scanning each of the three luminescent color strips.

A tube of this type is infinitely more simple to produce than the shadow-mask tubes which have most frequently been employed up to the present time and makes it possible to obtain a reproduction of color images of considerably superior quality with much more simple means. In particular, it can very readily be ensured in accordance with the invention that the three electron guns corresponding to the light signals of the three primary colors are constrained to follow the displacements of the master gun in such a manner that their three spots should follow on the screen rectilineal paths which are parallel to those of the spot of the master gun: it is in fact only necessary to carry out before putting the tube into service a permanent adjustment of the centering permanent magnets placed around the neck of the picture tube by producing action on the internal pole-pieces in such a manner that the deflections produced by these latter should cause the four beams corresponding to the four electron guns to penetrate into the zone in which the magnetic scanning fields are applied as parallel as possible to the axis of the deflecting system and as close as possible to said axis. Providing that this condition is satisfied, the deflections applied to the master beam by means of the orthogonal magnetic fields are automatically followed by the three slave beams. Finally, it is important to note that the luminescent screen tube in accordance with the invention ensures compatibility with respect to black and white transmissions; it is only necessary for this purpose, as .in the case of shadow-mask tubes, to adjust once and for all the ratio of currents of the three electron guns.

In a particular embodiment of the luminescent screen tube for reproduction of color television images, the device is simplified by making use of only two electron guns. The first gun accordingly performs the function of master gun and emits the guiding beam, the spot of which is caused to follow the rectilineal and horizontal guiding strips on the screen; the other electron gun alone carries out, as a result of an additional instantaneous vertical deflection imparted thereto with respect to its position of dependence on the mastergun, a point-to-point scanning of the three luminescent strips corresponding to the three primary colors. This result is obtained by means of a ternary sequence which controls simultaneously a pair of electrodes for additional vertical deflection provided in the tube at the level of the deflecting coils as well as the successive switching of the three luminance signals. This form of construction results in a tube of more simple design but it remains apparent that, with respect to the previous embodiment in which three electron guns are each specialized in one color, there is a loss of a certain quantity of information and that the overall restitution of the image is of inferior quality inasmuch as a loss of definition occurs in the horizontal direction.

Finally, and in a general manner, the luminescent screen tube in accordance with the invention for the reproduction of color television images is wholly suited to the method of scanning which is usually employed in practice and known as altemate-field scanning or double interlacing. This method of scanning has been developed in order to make it possible without increasing the quantity of information transmitted, to eliminate effects of flicker which resulted from an excessive time interval between two consecutive images. In a system of this type, scanning of horizontal lines is carried out by dividing each image into two half-images, each consisting of only even-numbered lines or oddnumbered lines. Many alternative forms of construction can be employed for adapting the luminescent screen tubes according to the present invention to the aforesaid method of interlaced scanning; particular mention can nevertheless be made of two such forms of construction which appear to be among the most attractive. 

1. A method of guiding the beam of charged particles of a luminescent screen display tube, said beam being displaced under the combined action of two orthogonal deflection scans to scan the screen of said display tube in a sequence of lines distributed over the screen, said lines being traced by a spot where said beam in an unblanked state impinges on the screen, said method comprising the steps of: deflecting the beam prior to beginning the scan of each line of said sequence to a portion of said screen beyond the display area thereof, hereafter referred to as the line entry guide area of the screen; detecting the beam spot position relative to a continuation in said guide area of a line of said sequence, by differential excitation by the beam of two portions of said guide area on eiTher side of said line continuation and using a signal produced by such detection to modify the beam deflection to guide said spot towards said line continuation while the scanning movement for scanning the corresponding line begins, and scanning said line subject to similar modification of the deflection transverse to said line pursuant to detection of differential excitation of guiding stripes on said screen on either side of said line, the effective range of deviation detectable as aforesaid and correctible by such modification being much less in case of said line than in case of the extension thereof in said line entry guide area.
 2. A method according to claim 1 in which the step of detecting differential excitation is carried out to produce a signal corresponding to the differential excitation ratio rather than to an arithnetic excitation difference, which signal is then applied to modify the deflection of the beam.
 3. A method according to claim 1 in which during the line scan the progress of said spot is detected by means of at least one of said guide stripes to produce a signal applied to the line scan deflection to accelerate or decelerate the scan so as to maintain the scan velocity constant.
 4. A method according to claim 1 in which the step of detecting differential excitation is carried out by comparing the amplitude of two frequencies produced by the scanning of said guide stripes by said spot as the result of the respective patterns of said guide stripes in the form of width modulation along their length at two respective frequencies.
 5. A method according to claim 1 which includes the step of increasing the intensity of the beam during at least part of the time the said spot is in said line entry guide area.
 6. A method according to claim 1 in which a plurality of beams are deflected with the use of common deflection means to scan lines slightly displaced from each other, in which further only one of said beams is deflected in such a path that its spot impinges on said guide stripes, and in which the said modification of deflection affects all of said beams equally.
 7. A method according to claim 6 in which the beam which is deflected so that its spot scans said guide stripes, is modulated at a predetermined frequency by the effect of which the excitation of said guide areas and said guide stripes by that one of said beams is distinguished from the effects of the other or others of said beams.
 8. A method according to claim 1 in which that one of said beams which is deflected so that its spot scans said guide guide stripes, is unblanked while deflected into said line entry guide area.
 9. A luminescent screen display tube having a luminescent screen including a display portion and a smaller adjacent guide portion and having at least one electron gun means for producing a deflectable electron beam for scanning said screen, said screen being provided over its entire display portion with a series of pairs of guide strips, each pair corresponding to one of the lines of a scan pattern over which the beam of one said gun of the tube is adapted to be scanned, the spacing between the two strips of a pair being smaller than the diameter of the spot produced by the impingement of said beam on said screen, said guides being arranged to produce a detectable excitation upon impingement of said beam in proportion to the area of impingement and being patterned with a width modulation along their length to produce modulation of said excitation during scan, the width modulation of two guides of a pair being at a different pitch.
 10. A luminescent screen display tube according to claim 9, wherein the upper and lower guide strips of each pair are constituted by a series of broken lines interrupted by regularly spaced gaps with a pitch that is different as between the two strips of a pair.
 11. A luminescent screen display tube according to claim 9, wherein the width-modulated strips are metallic and the two guide strips of eaCh pair are electrically connected and constitute a a single electrode, whereby the fraction of the beam current containing components which are modulated at the frequencies corresponding to the scanned width modulation may be used to correct the beam deflection.
 12. A luminescent screen display tube according to claim 9, wherein the width-modulated strips are formed of a substance which gives rise under the action of the spot to the emission of an electromagnetic radiation and wherein photoelectric cell means are provided in the proximity of the tube neck for selective detection of such radiation and formation of a signal therefrom.
 13. A luminescent screen display tube according to claim 9, wherein the width-modulated strips are formed of a substance which gives rise under the action of the spot to the emission of a secondary-electron current and means are provided within said tube for detection of said current and obtaining a signal therefrom.
 14. A luminescent screen display tube according to claim 9 for the reproduction of color television images by the three-color process, wherein the guiding strips are each associated with a triplet of three continuous horizontal and rectilinear strips of luminescent material each corresponding to one of the primary colors, and wherein the tube is provided with four electron guns in which a first gun thereof is a master gun and emits a beam impinging on said guiding strips and wherein the three other electron guns or so-called slave guns each correspond to the three primary colors and emit beams which are caused to follow the displacements of the master beam, by being subjected to common deflecting forces, so that corresponding spots move parallel to the guiding beam spot and scan with strict accuracy each of the three luminescent color strips respectively.
 15. A luminescent screen display tube according to claim 14 having associated therewith means for producing magnetic scanning deflection of the beams emitted by said four electron guns and having also means for producing constant centering magnetic fields affecting said beams in such a way that said beams penetrate into the zone in which said magnetic scanning deflection is applied while proceeding as parallel as practically possible to the axis of the tube and as close as practically possible to said axis.
 16. A luminescent screen display tube according to claim 9 adapted for the reproduction of color television images in accordance with the method of horizontal scanning known as interlaced scanning, wherein the guiding strips are each associated with a plurality of luminescent color strips and only some of said strips are scanned by the beams at each half-field.
 17. A luminescent screen display tube according to claim 16 in which each guide strip pair is disposed in predetermined relation to two triplets of luminescent color strips such that when said guide strip pair is scanned in one half frame one of said triplets will be scanned, and when said guide strip pair is scanned in the other half frame the other of said triplets will be scanned.
 18. A luminescent display apparatus including a luminescent screen tube as defined in claim 9 and also including means for deflecting said electron beam as aforesaid and means responsive to the excitation of said guides for correcting the deflection of said electron beam in a direction transverse to said scan lines, said apparatus further including means for detecting variation in frequency or phase of one of the frequencies of variation or of said detectable excitation produced by the scanning of said guides by said spot and for utilizing said frequency or phase variation to modify the deflection of said electron to make the velocity of scan more uniform.
 19. A luminescent screen display apparatus including a luminescent screen display tube according to claim 9 adapted for the reproduction of color television images by the three-color process, wherein said guiding strips are each assOciated with a triplet of three continuous horizontal and rectilinear strips of luminescent material each corresponding to one of the primary colors, and wherein the tube is provided with two electron guns, said apparatus including means for deflecting the beams produced by said guns, means responsive to the excitation of said guiding strips by one of said guns, referred to as the master gun, for modifying the operation of said deflecting means to assure the guidance of the beam of said master gun with reference to said guiding strips and means for causing the beam of the gun other than said master gun, referred to as the slave gun, to be rapidly deflected vertically relative to the beam of said master gun, so that the beam of said slave gun may scan the three luminescent strips of said triplets in a sequence suitable for coordination with successive switching of color video signals.
 20. A luminescent screen display tube having a luminescent screen including a display portion and a smaller adjacent guide portion, and having at least one electron gun means for producing a deflectable electron beam for scanning said screen, said screen being provided over its entire display portion with a series of pairs of guide strips, each pair corresponding to one of the lines of a scan pattern over which the beam of one said gun of the tube is adapted to be scanned, the spacing between the two strips of a pair being smaller than the diameter of the spot produced by the impingement of said beam on said screen, said guides being arranged to produce a detectable excitation upon impingement of said beam such that the excitation of one guide of a pair can be distinguished from that of the other guide of said pair, said guide area of said screen being adjacent to its display area along a boundary to which the ends of all the scan lines of said display area begin, said guide area of said screen including principally pairs of guide zones such that each member of a pair of said zones is separated by the extension of a scan line into said guide area of said screen, said guide zones being arranged to provide a detectable excitation when said beam impinges thereon which is not distinguishable from that of the adjacent guide stripe, but is distinguishable from that produced by the guide stripe just across said scan line from said adjacent guide stripe.
 21. A luminescent screen display tube according to claim 20 in which said guide stripes and said guide zones are discontinuous in the direction of the adjacent scan line, in the sense that there are gaps in which the presence of the spot of said beam does not produce the aforesaid detectable excitation produced by the presence of said spot on other portions of said guide stripes and guide zones, and in which said gaps are provided at regular intervals in the direction of said scan line with a pitch on one side of said scan line or its extension in said guide area different from the pitch of said gaps on the other side of scan line or its extension in said scan area.
 22. A luminescent screen display tube according to claim 21 adapted for the reproduction of color television images in accordance with the method of line scanning known as interlaced scanning, wherein the aforesaid guide strips are so provided that adjacent pairs of guide strips have a reversed relation of the relative characteristics of the strips of the pair.
 23. A luminescent screen display tube according to claim 20 for the reproduction of color television images by the three-color process, wherein the guiding strips are each associated with a triplet of three continuous horizontal and rectilinear strips of luminescent material each corresponding to one of the primary colors, and wherein the tube is provided with four electron guns in which a first gun thereof is a master gun and emits a beam impinging on said guiding strips and wherein the three other electron guns or so-called slave guns each correspond to the three primary colors and emit beams which are caused to fOllow the displacements of the master beam, by being subjected to common deflecting forces, so that corresponding spots move parallel to the guiding beam spot and scan with strict accuracy each of the three luminescent color strips respectively.
 24. A luminescent screen display tube according to claim 23 having associated therewith means for producing magnetic scanning deflection of the beams emitted by said four electron guns and having also means for producing constant centering magnetic fields affecting said beams in such a way that said beams penetrate into the zone in which said magnetic scanning deflection is applied while proceeding as parallel as practically possible to the axis of the tube and as close as practically possible to said axis.
 25. A luminescent screen display apparatus including a luminescent screen display tube according to claim 23 adapted for the reproduction of color television images by the three-color process, wherein said guiding strips are each associated with a triplet of three continuous horizontal and rectilinear strips of luminescent material each corresponding to one of the primary colors, and wherein the tube is provided with two electron guns, said apparatus including means for deflecting the beams produced by said guns, means responsive to the excitation of said guiding strips by one of said guns, referred to as the master gun, for modifying the operation of said deflecting means to assure the guidance of the beam of said master gun with reference to said guiding strips and means for causing the beam of the gun other than said master gun, referred to as the slave gun, to be rapidly deflected vertically relative to the beam of said master gun, so that the beam of said slave gun may scan the three luminescent strips of said triplets in a sequence suitable for coordination with successive switching of color video signals.
 26. A luminescent screen display tube according to claim 20 adapted for the reproduction of color television images in accordance with the method of horizontal scanning known as interlaced scanning, wherein the guiding strips are each associated with a plurality of luminescent color strips and only some of said strips are scanned by the beams at each half-field.
 27. A luminescent screen display tube according to claim 20 in which each guide strip pair is disposed in predetermined relation to two triplets of luminescent color strips such that when said guide strip pair is scanned in one half frame one of said triplets will be scanned, and when said guide strip pair is scanned in the other half frame the other of said triplets will be scanned. 